1. Field of the Invention
The present invention relates to a variable valve timing control apparatus that adjusts valve timing of an internal combustion engine.
2. Description of Related Art
In a conventional hydraulic variable valve timing unit, as shown in JP-A-H9-324613 (corresponding to U.S. Pat. No. 5,738,056) and JP-A-2001-159330 (corresponding to U.S. Pat. No. 6,330,870), a lock phase during engine stop is set at a generally middle phase within an adjustable range of a camshaft phase such that the adjustable range of valve timing (camshaft phase) is enlarged. In the above conventional art, an intermediate lock phase, at which the phase is locked during the engine stop, is set at a phase suitable for starting the engine. The engine is started while the camshaft phase is at the intermediate lock phase. Also, after starting the engine, the lock is released such that valve timing (camshaft phase) is shifted toward a target phase in the retard direction or in the advance direction. Then, when the engine is stopped, the camshaft phase is controlled at a position on an advance side of the intermediate lock phase. In the above state, oil pressure supplied to the variable valve timing unit is reduced such that the camshaft phase is shifted in the retard direction by cam torque. When the camshaft phase reaches the intermediate lock phase during the process of the shift, a spring pushes a lock pin into a lock hole, and thereby the camshaft phase is locked at the intermediate lock phase.
In the conventional variable valve timing control apparatus, it is determined that the lock has been completed if the camshaft phase stops around the intermediate lock phase in a process, where the camshaft phase is shifted in the retard by the cam torque when the engine is controlled to be stopped. As a result, in a case, where the camshaft phase erroneously stops at a certain phase slightly different from the intermediate lock phase due to some reasons (for example, a condition of oil temperature), it may be erroneously determined that the lock has been completed although the camshaft phase is not locked actually.
Also, in the conventional art, the lock is released such that the camshaft phase is shifted in the retard direction or in the advance direction after the completion of the engine start. However, there may be a case, where the release of the lock is delayed due to some reasons (for example, frictional force between the lock pin and an inner periphery edge of the lock hole, or delay in the increase of oil pressure). In the usual operation, when the lock release request is issued, the camshaft phase is phase feed-back controlled to be shifted in the retard direction or in the advance direction based on the target phase while the lock pin is actuated by the oil pressure in the lock release direction. In the above operation, because of the above delay of the lock release, the lock pin may be firmly urged against the inner periphery edge of the lock hole by the driving force, which shifts the camshaft phase toward the target phase, while the lock pin is being pulled out of the lock hole. As a result, the lock pin is disabled to get out of the lock hole, and thereby the lock may be prevented from being released disadvantageously.